Generally, due to excessive use of underground water in a coastal area, the underground water level descends and this causes saltwater (sea water) to intrude into the underground water and contaminates the underground water so that the underground water in the coastal area may not be used as agricultural water or industrial water, let alone drinking water.
Therefore, in such a coastal area, underground water observation wells are typically used. To form such an underground water observation well, an observation well is drilled and a wire or wireless sensor for measuring the water level, the temperature and the electrical conductivity of the underground water is installed in the observation well so as to monitor a change of the properties of the underground water.
The freshwater/saltwater interface (the interface between underground water and saltwater) may vary according to two mechanisms. First, the interface may vary according to a reduction in the thickness of a freshwater layer. Second, the interface may vary according to a change of the thickness of a saltwater layer.
The thickness of the freshwater layer may increase by recharge of rainfall or may decrease (become thin) by water pumping in an area around the freshwater.
The thickness of the saltwater layer varies according to a periodical variation in the height of the sea water surface that varies by the rising and falling of the tide.
In recent years, the sea water surface has gradually risen as a result of the global warming phenomenon.
When the thickness of the freshwater layer becomes thin and the thickness of the saltwater layer becomes thick compared to a reference point according to various phenomena, the freshwater/saltwater interface rises. In contrast, when the thickness of the freshwater layer becomes thick and the thickness of the saltwater layer becomes thin, the freshwater/saltwater interface falls.
As described above, according to various and complex phenomena, such as the pumping of excessive amounts of underground water, the recharge of rainfall, the rising and falling of the tide and the rising of the sea water surface by global warming, the location of the freshwater/saltwater interface may vary at any time.
However, the measuring sensor that is installed in the underground water observation well of the coastal area is located at a fixed depth regardless of wire or wireless type so that the sensor may not determine the precise location of the freshwater/saltwater interface in real time. Although the sensor determines the location of the freshwater/saltwater interface in real time, the sensor cannot monitor a change of the freshwater lens thickness or a change of the freshwater/saltwater interface depth in real time so that see wate may infiltrate into the underground water or coastal aquifer.